This invention concerns receiver sheets, particularly transparent receiver sheets adapted to receive a colour toner image by an electrophotographic or electrostatic process and suitable for use in an overhead projector (OHP).
Overhead projectors project an image by passage of light through a transparent film, known as a receiver sheet, bearing the image to be projected. The image is formed or the film by electrophotography or electrostatic printing. The receiver sheet is placed into an electrophotographic or electrostatic printer or copier and toner (black for a black and white image, or a series of colour toners (CMYK (cyan, magenta, yellow, black)) for a full colour image) is deposited imagewise and fused into the receiver surface to form an image on the receiver sheet.
For black and white OHP purposes, OHP receiver sheets are usually in the form of polyester such as biaxially oriented poly(ethylene terephthalate) (PET) base film typically about 100 xcexcm thick (eg Melinex (Melinex is a Trade Mark) from ICI/Dupont) with a coating on the surface to promote adhesion of the toner to the base film. The coating is typically of an acrilic polymer and is either applied during the manufacture of the PET film or is coated off-line, i.e. as a separate step. The coating usually also contains filler to improve the handling properties and some Aintistatic agent (sometimes applied as an extra coating known as a supercoat). Such materials work well in black and white OHPs, where the aim of the toner is simply to block out projected light and hence project a dark area. Different considerations apply, however, in projecting colour images on an OPP. In this case light must be transmitted through a coloured block and accurately transmit the correct spectrum of light onto a screen. This must be achieved with minimum loss due to light scatter. It is therefore necessary for the coloured block on the film to be as transparent as possible. If the toner particles stick to the OHP surface and remain proud of the surface, there will be a haze in the film and scattering of the light at the surface of the film. This will cause the projected image on the screen to look dull and rather grey.
If conventional black and white OHP receiver sheets are used for colour OHPs, poor results are obtained. The present invention aims to provide a novel receiver sheet better suited to use with colour OHPs.
According to the present invention there is provided a transparent receiver sheet adapted to receive a colour toner image by an electrophotographic or electrostatic process and suitable for use in an overhead projector, comprising a toner acceptance layer that comprises a resin condensation product of toluene sulphonamide.
The resin is conveniently a condensation product of toluene sulphonamide and formaldehyde.
The resin suitably has a melting point of at least 60xc2x0 C.
Suitable resins may be made by known techniques, or are available commercially. One preferred commercially available resin is Ketjenflex MH (Ketjenflex MH is a Trade Mark) from Akzo, which has as its main functional component a resin condensation product of toluene sulphonamide and formaldehyde present in the form of a mixture of mainly dimer, trimer and tetramers. Ketjenflex MH has the desirable properties of being compatible with a wide range of polymer classes (which is of importance for reasons to be discussed below), having a low melting point (about 62xc2x0 C.) which is very sharp, and having a low melt viscosity. Ketjenflex MH has a glass transition temperature (Tg) of 40xc2x0 C.
The resin preferably constitutes an amount in the range 40 to 100% by weight of the weight of the toner acceptance layer.
The toner acceptance layer optionally includes one or more secondary polymers, typically in an amount in the range of 0 to 60% by weight of the weight of the toner acceptance layer. The secondary polymer preferably has a molecular weight as low as possible compatible with good film-forming properties, with the molecular weight typically being in the range 1000 to 20000. Low molecular weight is also important in reducing the melt viscosity to a value that allows easy penetration of toner, in use. The lower the molecular weight of the secondary polymer, the better the fusion performance as discussed below. However, low molecular weight polymers may present manufacturing difficulties, so a compromise balance should be reached. The secondary polymer is preferably selected for compatibility with the resin and with the toner with which the sheet is intended for use. Thus, the secondary polymer desirably has a melting point that is similar to or slightly higher than that of the resin. The secondary polymer advantageously has a chemical structure and/or chemical properties and/or solubility parameters similar to those of the toner to ensure compatibility on blending. Compatibility between secondary polymer, resin and toner is also important to reduce light scattering as far as possible. Further, the melting point of the secondary polymer is desirably sufficiently low to ensure efficient fusing of toner particles into the surface of the toner acceptance layer in use.
The secondary polymer desirably has a Tg below 60xc2x0 C.
For good performance in use, a number of factors need to be controlled, as follows:
a/ Compatibility
The polymers/resins in the acceptance layer must be compatible with polymer in the toner particles. This can be controlled by using materials with similar chemical properties and/or similar solubility parameters. Compatibility can be improved with the use of low molecular weight resins/polymers.
b/ Melting Point
For the fusing of the compatible materials, the melting point of the toner acceptance layer must be similar to or slightly lower than that of the toner. The temperature that the toner is exposed to during production of an image is above its melting point, which allows it to adhere to the receiver sheet. If both the toner acceptance layer and the toner melt at the same time, they have the possibility of mixing and forming a coherent fused layer.
c/ Melt Viscosity
For the toner acceptance layer and toner particles to become fused into one layer, the two must be able to mix rapidly. To ensure this occurs, the melt viscosity of the acceptance layer must below. However, if the melt viscosity is too low or if the melting range of the acceptance layer is smooth, rather than sharp, then there is a possibility that the toner particles can spread too far and produce a blurred image. The acceptance layer should therefore have a low melt viscosity but with a sharp melting point.
While a toner acceptance layer comprising resin alone functions well in use providing the electrical characteristics are sufficient to accept charged toner, secondary polymer is preferably included to facilitate manufacture and improve the handling properties of the layer.
Good results have been obtained using secondary polymer selected from the following.
(a) Vinyl chloride/vinyl acetate (VC/VA) copolymer, eg Vinylite VYES-4 (Vinylite VYES-4 is a Trade Mark) from Union Carbide.
(b) Polyesters, eg Vylon 200 (Vylon 200 is a Trade Mark) from Toyobo, and the amorphous polyesters Dynapol 850, Dynapol 411 and Dynapol 206 (Dynapol is a Trade Mark) from Allchem/Huls.
(c) Fumarate polymers, eg Atlac 363E (Atlac 363E is a Trade Mark) from DSM Resins, which is a propoxylated bisphenol A fumarate polymer, with unsaturation on the fumarate ester group still present.
(d) Epoxy resin, eg Epikote (Epikote is a Trade Mark) from Shell, which is a bisphenol A epoxide.
(e) Acrylates, eg quarternised acrylates such as Surcol 860 and Surcol SP6 (Surcol ia a Trade Mark) from Ciba Speciality Chemicals.
It is expected that similar results would be obtained with a wide range of other secondary polymers including polymers including polyurethanes and polystyrenes.
Mixtures of secondary polymers may be used.
The toner acceptance layer desirably also includes a minor amount of particulate filler material, typically constituting an amount in the range 0.01 to 2% by weight of the weight of the toner acceptance layer. Inclusion of filler material gives the sheet a degree of surface roughness and prevents the surface of the sheet from being so smooth that it adheres to other smooth surfaces such as another similar sheet or a fuser roller. The particulate filler material conveniently has a particle size in the range 1 to 8 xcexcm, preferably 1 to 5 xcexcm, and may be of materials such as silica, particularly porous silica, polyethylene, or resin such as urea-formaldehyde resin. Good results have been obtained using, as filler materials, Syloid 244 (Syloid 244 is a Trade Mark) from Grace, which is a high porosity synthetic silica milled to a particle size of about 2 xcexcm, Pergopak M3 (Pergopak M3 is a Trade Mark) from Omya Croxton and Gary, which is a milled urea-formaldehyde resin material with a particle size of about 3 xcexcm, and MMP635F polythylene filler (MMP635F is a Trade Mark) from Micro Powders Inc. Mixtures of filler materials may be used.
It is found in practice that certain polymeric filler materials, including polyethylene fillers, may melt during the printing process so that the printed sheet no longer has a surface roughness. This is advantageous, as surface roughness would otherwise cause undesirable haze.
The toner acceptance layer desirably includes one or more anti-static materials with the aim of avoiding the need to use a supercoat. Suitable anti-static materials include one or more of lithium hydroxide and toluenesulphonic acid. Lithum ions provide both humectant properties and ionic conductivity. Diethylene glycol may also be included to assist these functions. Other anti-static materials include quarternised acrylates, nonyl phenol ethoxylate, sodium alkyl sulphonate and stearamide propyl methyl xcex2-hydroxy ethyl ammonium di-hydrogen phosphate.
Flow agents such as silicones or acrylic ester polymers may optionally be included, particularly in conjunction with anti-static materials, to improve performance.
A supercoat may be provided in known manner on the toner acceptance layer if appropriate, eg to modify the surface conductivity.
The receiver sheet typically further comprises a base sheet of transparent material, usually plastics material, carrying the toner acceptance layer. The base sheet is conveniently made of polyester, eg poly(ethyleneterephthalate) (PET) such as Melinex or poly(ethylenenaphthalate) (PEN). The base sheet may alternatively be of polycarbonate. The base sheet typically has a thickness in the range 50 to 150 xcexcm, eg about 100 xcexcm. The base sheet may be pre-treated with an adhesion-promoting priming layer, eg of parachlorometacresol (PCMC), prior to application of the toner acceptance layer.
The receiver sheet will typically include one or more opaque portions for location by a photoelectric detector of a printer to facilitate correct feeding of the sheet, in known manner. Such an opaque portion may be produced by a pigmented or scattering stripe along one or more edges. Alternatively a strip of plain paper may be temporarily laminated behind the receiver sheet.
Typical examples of toner acceptance layer formulations are as follows, expressed in terms of parts by weight. Filler is expressed as wt % based on the total weight of polymers (resin and secondary polymer).
1/
Ketjenflex MH 50 parts. Range (40-100 parts) Preferred range (50-80 parts)
Vinylite VYES-4 50 parts. Range (0-60 parts) Preferred range (20-50 parts)
Pergopak M3 filler 0.3 parts on polymers. Range (0.01-2 parts) Preferred range (0.1-0.6 parts)
2/
Ketjenflex MH 85 parts. Range (70-100 parts) Preferred range (80-95 parts)
Vylon 200 15 parts. Range (0-30 parts) Preferred range (5-20 parts)
Pergopak M3. 0.3 parts on polymers. Range (0.01-2 parts) Preferred range (0.1-0.6 parts)
3/
Ketjenflex MH 70 parts. Range (40-100 parts) Preferred range (50-90 parts)
Atlac 363E 30 parts. Range (0-60 parts) Preferred range (10-50 parts)
Pergopak M3. 0.3 parts on polymers. Range (0.01-2 parts) Preferred range (0.1-0.6 parts)
4/
Sample formations including anti-static materials are as follows:
Vinylite VYES-4 20-70 parts (Preferably 30-50)
Ketjenflex MH 80-30 parts (Preferably 70-50)
Pergopak M3. 0.05-1 part (Preferably 0.1-0.6)
Digol (Diethylene Glycol). 3-20 parts (Preferably 5-15)
Toluene 4 Sulphonic acid. 3-20 parts (Preferably 5-15)
Lithium Hydroxide. 1-6 parts (Preferably 2-4)
In all cases, Pergopak filler can be replaced by a silica filler, such a Syloid 244. etc.
The toner acceptance layer ingredients are usually dissolved in a suitable solvent or solvent system (mixture of solvents) which can be any solvent or mixture of solvents that dissolve the materials and that is compatible with the other materials involved and coating technique used.
Suitable solvents include one or more of methanol, n-butanol, methyl ethyl ketone, acetone and toluene. A suitable solvent system for all the above formulations is 10 parts by weight methanol/90 parts by weight acetone.
Currently favoured formulations comprise Ketjenflex MH and amorphous polyester (Dynapol L411), typically at a weight ratio in the range 3:1 to 1.1, preferably 1.5:1. The preferred solvent system for such formulations comprises a mixture of acetone and methyl ethyl ketone, possibly with a small amount of toluene. The preferred anti-static material in such formulations is Atmer 190 (20% solution) and/or Alcosist PN, with the preferred filler material being Pergopak M3.
The receiver sheet may be produced by coating, eg by a roller coating technique, a solution of the toner acceptance layer ingredients, ie resin, optional secondary polymer, filler, anti-static materials etc, dissolved in a suitable solvent system, eg 10 parts by weight methanol/90 parts by weight acetone, onto a base sheet possibly pretreated with an adhesion-promoting priming layer, eg of PCMC, followed by drying to produce the toner acceptance layer. A supercoat may be optionally provided on the toner acceptance layer. An opaque location portion may be included along one or more edges.
In a further aspect the invention thus provides a method of manufacturing a transparent receiver sheet adapted to receive a colour toner image by an electrophotographic or electrostatic process and suitable for use in an overhead projector, comprising providing on a base sheet of transparent material a toner acceptance layer that comprises a resin condensation product of toluene sulphonamide.
The receiver sheet may be used in conventional manner to receive a colour image for use in an OHP. This generally involves transferring colour toner particles to the toner acceptance layer, eg using a colour xerographic copier or printer in a process that typically involves passing the receiver sheet through heated rollers to fuse the toner particles to each other and to ensure firm attachment to the receiver sheet. Silicone oil is typically used as a lubricant to ensure that the toner particles do not adhere to the heated rollers.
In another aspect the invention thus provides a method of producing a sheet for projection of colour images by an overhead projector, comprising forming a colour toner image on a receiver sheet in accordance with the invention.
It is found that with receiver sheets in accordance with the invention, the properties of the resin can provide good compatibility with toner particles in current use, such that the toner particles fuse into the toner acceptance layer, resulting in transparent colour blocks with low haze and good clarity. When used in an OHP, projected images that are bright in colour can be obtained from such sheets. The good compatibility of the acceptance layer with the toner particles gives good projected optical densities A secondary benefit is good toner adhesion, which is of importance to end users when assessing receiver sheets and handling OHPs.